This project is designed to delineate sites and mechanisms of hormonal control of renal sodium transport. Initially, proximal straight tubules (PSTs) from rabbits have been studied using the technique of isolated renal tubule perfusion developed in this laboratory. Early PSTs from superficial nephrons of rabbits treated chronically with deoxycorticosterone acetate (DOCA) prior to sacrifice had a significantly higher rate of iosotonic fluid reabsorption than the same segment from untreated rabbits. In addition photo-micrographs reveal that the cells are thicker and the outer tubule diameters are greater in the DOCA-treated animals.